Reduction in thickness and size and multi-functionalization of information terminal devices have rapidly proceeded. In the circumstances, thinner and denser semiconductor devices, which are to be installed in the information terminal devices, are also required. For reducing thickness of the semiconductor devices, a semiconductor wafer on which semiconductors are to be mounted, is required to reduce thickness. To deal with the requirement, the back surface of a semiconductor wafer is ground to reduce thickness.
Recently, the back surface of a semiconductor wafer having uneven portions such as bumps (electrodes) of about 30 μm to 100 μm in height, formed of solder etc., has been occasionally ground. In grinding the back surface of such a semiconductor wafer having bumps, a back grinding sheet (hereinafter, referred to also as “BG sheet”) is generally stuck for protecting the surface of the uneven portions such as bumps.
However, when a BG sheet generally used is stuck to a semiconductor wafer having bumps, the BG sheet on portions where bumps are present differs in height from that on portions where bumps are not present. As a result, it is difficult to obtain a flat surface of the wafer after the BG sheet is stuck. Because of the difference in height of the wafer surface, pressure is unevenly applied to the back surface of the wafer. As a result, when the back surface of the wafer is ground, dimples (chips, dints) and cracks (breakage) are produced. As the case may be, the semiconductor wafer is occasionally broken.
To overcome such a problem, for example, Patent document 1 discloses a surface protecting sheet for a semiconductor wafer, having a resin layer, which is adjusted to have storage elastic moduli at 25° C. and 60° C. within predetermined ranges. The surface protecting sheet for a semiconductor wafer overcomes the above problem by providing a resin layer having a difference in storage elastic modulus between room temperature (25° C.) and high temperature (60° C.), more specifically, by sticking the resin layer to uneven portions such as bumps of a wafer at a high temperature, thereby softening the resin layer, absorbing uneven portions such as bumps and reducing difference in height of the wafer surface.